Trouble holding tolerance

[RickP]

We are turning a 127mm part in a double headed center drive lathe. The part sticks out of the chucks about 500mm. The part is first bored and then supported by a tailstock. The OD tolerance is +/- 0.019mm. The CPk for this process is not acceptable. When we run 25 parts 22-23 are excellant and within tolerance. If these were the only parts the process would have an acceptable CPk. The problem is the other 2-3 parts. They are flyers and are both over or under tolerance by 0.1mm or more. Can anyone help with a list to things to check to find the source of the variation? I am leaving off the list of items we already checked so as to not limit the ideas you many have.

[ctate2000]

I do not think there is any way to slove this based on the limited information but if I had to guess, I would look first at the set up.



CT

[DMF_TomB]

0.025mm is about 0.001 inch and part is about 5 inch OD ?

What coolant are you using? A hot part expands oversize while hot and shrinks when it cools to room temperature.

Steel expands 0.000006 per inch per degree F
5" round expands 0.0003" per 10 degree F change. Temperature control is very important.

Other factor is poor surface finish causing a fuzzy surface that measures a certain size but quickly gets smaller when burnishing or even a quick light sanding.

Buildup of false cutting edge on tool bit. It will cut parts smaller with a temporary buildup stuck on tool edge/tip. Speed and feeds and coolant can have an important effect. Sometimes a higher speed and or feed will give a better surface finish and less of a buildup on tool edge / tip. Carbide inserts types can have a large affect.

A tool shape that applies a large force and cause spring effect which when doing a finish cut you might measure needing a 0.005" additional cut but it cuts more, for example 0.006" because the last rough cut of 0.100+ was too much of a change. I usually find planning on 2 or 3 finishing cuts better for higher accuracy. I sometimes take cuts of only 0.001" or even 0.0005". If finish looks poor and you are only 0.0005" oversize, try a little sanding.

Hard to say your tolerance holding problem is. Also some people have trouble measuring to less than 0.025mm repeatedly, that is measuring a part 10 times and getting different readings +/- 0.01 mm